This invention relates to electro-mechanical cables for cable deployed (cable suspended) pumping systems, and is more particularly concerned with a cable construction that imposes substantial restraint against outward radial expansion of a cable core without imposing substantial axial restraint, and that prevents kinking due to radial expansion.
Over the last two decades there has been substantial progress in perfecting down hole equipment for cable deployed pumping systems. Reliability of the down hole equipment has improved substantially and has been confirmed by extensive testing. Nevertheless, the electro-mechanical cable for deploying the down hole equipment and for providing electrical power has remained a weak link in the system.
Electro-mechanical cables for use in cable deployed pumping systems require special properties that are not found in ordinary electrical cables. For example, a #1 size three-conductor electrical cable can normally support tensile loads of only 2,000 to 4,000 lbs , but a #1 electro-mechanical cable for use in cable deployed pumping systems must be capable of supporting tensile loads in excess of 100,000 lbs.
One of the problems that has plagued electro-mechanical cables for cable deployed pumping systems is kinking of the cable due to thermal expansion. Prior attempts to solve this problem have required complex and expensive cable structures designed so as to physically separate the strength members of the cable from the electrical core.
Another problem is gas embolism due to rapid decompression of the cable after gases have dissolved in elastomeric materials of the cable. Rapid decompression occurs when down hole equipment is pulled from a well by means of the cable. In an effort to solve the embolism problem, the electrical core has been enveloped in a braid consisting of two layers of 132-0.014" dia. galvanized improved plow steel wires inter-woven in 12 bundles of 11 wires each wrapped at a neutral angle (e.g., about 53.degree.) that allows equal axial and radial expansion of the cable core. However, this cable construction has a kinking problem caused by thermal expansion of elastomeric cable insulation and jacket material interacting with steel armor wires that surround the braid. The following analysis underlying the invention is believed to explain this problem.
The cable can be considered to be in a zero stress condition during assembly, at a nominal temperature of 70.degree. F. When the cable is installed in a well whose temperature is substantially higher, the cable temperature will increase to the high well temperature. The elastomers in the core, which have a thermal expansion approximately ten times that of the steel armor wire, will increase in volume. The steel armor wire, due to its lay angle and orientation, will prevent any axial growth of the core, but the elastomers in the core will increase in diameter. The diameter of the armor will increase, but the length of the armor wire will stay constant, causing the length of the cable to decrease. This applies compressive stress to the cable, which buckles the copper conductors, and ultimately causes an electrical failure.